In recent years, microelectromechanical system (MEMS) sensors have been widely used in consumer electronics. For example, MEMS motion sensors are used in mobile phones and MEMS gyroscopes are used in interactive games. Therefore, MEMS sensors need to have dominant competitiveness such as small size or low manufacturing cost in order to enter the market of consumer electronics product.
In the product launch of Year 2010, Apple Inc. has officially announced that iPhone 4G cell phone is built-in a tri-axial MEMS gyroscope (“MEMS-Gyro”) so that the cell phone has the function with more realistic interactive game. Such trend represents that the MEMS-Gyro market will enter a period of high-speed growth. Therefore, high sensitivity and low cost will be the competitive advantages in the high-speed growing MEMS-Gyro market.
FIG. 1A is the top view of the simplified schematic of a traditional gyroscope and FIG. 1B is a cross sectional view taken along the line 1-1 in FIG. 1A. The gyroscope 10 comprises a substrate 16, a proof mass 11, a plurality of first springs 12, a frame 13, a plurality of second springs 14 and a plurality of anchors 15. The plurality of first springs 12 are connected to the proof mass 11 and the frame 13. The frame 13 surrounds the plurality of first springs 12 and the proof mass 11. The plurality of second springs 14 are connected to the frame 13 and the plurality of anchors. The plurality of anchors are fixed on the substrate 16.
While the gyroscope 10 is active, a voltage is applied to the frame 13 and the proof mass 11 respectively, and then the frame 13 is driven to oscillate periodically along the X-axis, such that the proof mass 11 is driven to move along the X-axis. The electrical signal generated by the oscillation of the frame 13 is sensed by a movable comb structure (not shown) of the frame 13 and a fixed comb structure (not shown) on the substrate 16 and transmitted to an external ASIC chip. Then, the frame 13 is driven again by the circuit of the ASIC chip, and oscillates stably within the expected frequency range
If the gyroscope 10 senses angular velocity along the Z-axis, the proof mass 11 will move along the Y-axis at the same time. The movable comb structure (not shown) on the proof mass 11 and the fixed comb structure (not shown) on the substrate 16 generate another electrical signal due to the movement of the proof mass 11. The electrical signal will then be transmitted to the ASIC chip for calculating the angular velocity.
For driving the frame effectively, it is better to apply a higher voltage to the frame 13. At the same time, the lower voltage is applied to the proof mass for obtaining higher sensitivity of gyroscope. However, since the frame 13 and the proof mass 11 of the traditional gyroscope 10 use the same electrical path, as shown with the electrical paths a and b in FIGS. 1A and 1B, it is impossible to apply higher voltage to the frame 13 and to apply lower voltage to the proof mass 11 respectively.
U.S. Patents (U.S. Pat. No. 6,239,473, U.S. Pat. No. 6,433,401 and U.S. Pat. No. 6,291,875) disclose a proof mass with one independent electrical path or two independent electrical paths by using the insulating layer along the vertical axis (Z-axis), the isolating trench along the vertical axis (Z-axis) and the suspended structure. However, the suspended structure is formed by etching the lower portion of the structure to form the gap under the structure. Thus, it is impossible to create a relatively large suspended arm or suspended mass, such as proof mass of accelerometers or gyroscopes. In addition, due to the limitation of the process for forming the isolating trench, it is impossible that the isolating trench along the vertical axis (Z-axis) is formed on the springs 12 and 14 of the gyroscope 10 in FIG. 1. Therefore, the electrical signals of the proof mass 11 and the frame 13 will still be transmitted through the same electrical paths such as the springs 14 of the gyroscope 10.